In recent years, there has been an increasing demand for improving the fuel consumption of working machines such as hydraulic excavators. Various measures for meeting that demand have been proposed.
For example, there is proposed a hybrid hydraulic excavator that has an electric motor (generator) connected to a fixed displacement hydraulic motor attached to the hydraulic line (return oil hydraulic line) of the hydraulic chamber through which the return oil flows in a boom lowering operation on the bottom side of a boom cylinder (hydraulic actuators). This hybrid hydraulic excavator has the hydraulic motor driven by use of the return oil from the boom cylinder, the hydraulic motor in turn driving the electric motor. The electric energy obtained by driving the electric motor is stored into an electric storage device connected via an inverter, a chopper or the like.
As the power regeneration device for a working machine regenerating power by introducing the return oil from the boom cylinder into the fixed displacement hydraulic motor in the above-outlined manner, Patent Literature 1 describes one that branches the return oil from the boom cylinder into the power regeneration side (hydraulic motor side) and the control valve side so as to improve the operability of the hydraulic actuators.